When the metadata master starts up, it goes backwards through its per-period metadata logs until it finds a period with no log shards (stat returns ENOENT on all shards). It returns the period after that as it's oldest_log_period, and other zones start their sync at that point.

But it's possible and likely that intermediate periods only introduce realm configuration changes, and don't have any associated changes in the metadata log. So we'll skip past any logs that came before that, and advertise the wrong oldest_log_period to our peers.